Biaxially oriented films prepared from liquid crystalline thermoplastics are known and include, for example, biaxially oriented poly(p-phenyleneterephthalamide) prepared by J. E. Flood, J. L. White and J. F. Fellers, Journal of Applied Polymer Science, 27, 2965 (1982); biaxially oriented polyester from phenyl p-hydroxybenzoate, diphenyl isophthalate and hydroquinone prepared by U. Takanori, Y. Hirabayashi, M. Ogasawara and H. Inata, European Patent Application No. 24,499 (Jun. 24, 1981); and a 50:50 blend of thermoplastic liquid crystal polyester and poly(ethylene terephthalate) coextruded with poly(ethylene terephthalate) then biaxially stretched to produce a laminated film prepared by H. Minamizawa, I. Okazaki and K. Abe, Japan Kokai Tokkyo Koho JP 04,294,125 (Oct. 19, 1992).
U.S. Pat. No. 5,266,660 teaches the preparation of a uniaxially oriented coating on glass. For example, in Example 6, a curable mixture of the diglycidyl ether of 4,4'-dihydroxybiphenyl and sulfanilamide is prepared, then placement of a portion of the curable mixture between a pair of glass plates is followed by B-staging to develop a liquid crystalline state. Uniaxial orientation is induced via shearing of the liquid crystalline resin by moving one glass plate across the top of the resin. The uniaxially oriented domains which result are maintained with continued cure of the resin.
It would be desirable to have available biaxially oriented films from mesogen containing curable compositions containing an epoxy resin and a curing agent therefor. Generally, the relatively low molecular weight of the epoxy resin and curing agent leads to greatly enhanced processing relative to the much higher molecular weight thermoplastics. This ease of processing can manifest itself in reduced melting point, faster response to the force used to induce orientation, and the like. Furthermore, the thermoset mechanism can be used to lock in the biaxial orientation induced in the films of the present invention. This is in contrast to mesogen-containing thermoplastics where a lack of crosslinking can lead to eventual relaxation of the orientation of the mesogenic moleties contained therein. According to Friedhelm Hensen, ed., Plastics Extrusion Technology, published by Hanser Publishers, New York, New York (1988) on page 260, a problem similar to this is encountered in non-mesogenic oriented thermoplastics: "During drawing, energy conversion takes place. After drawing, entropy effects tend to return the aligned structure to its original configuration, causing the film to shrink."
Biaxial orientation, as opposed to uniaxial orientation is preferred because the lack of orientation in the transverse to the machine direction in a uniaxially oriented film induces very large differences in the mechanical properties in the machine and transverse to the machine directions. By way of contrast, in the films of the present invention, simultaneous biaxial drawing produces nearly equivalent mechanical properties in both the machine and transverse to the machine directions. Additionally, the use of a two-stage biaxial drawing process to manufacture the films of the present invention can provide different values in the two directions as a function of the order and extent of drawing, or also can be used to provide equivalent properties in the two directions. Aside from the improvement in one or more mechanical properties, such as, but not limited to modulus, exhibited by the biaxially oriented films of the present invention, certain of the cured films exhibit a reflective, metallic-like appearance as a direct result of the biaxial drawing.
The present invention provides a heretofore unknown class of mesogen containing biaxially oriented films possessing improvements in one or more physical and/or mechanical properties.